Pool Sanitizing: A Return to the Simple

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Photo courtesy of Shutterstock
Photo courtesy of Shutterstock


  • Affordable
  • Easy to use
  • Safe for storage as it is non-flammable and noncombustible
  • Leaves a measurable residual of free chlorine
  • Does not contain calcium or cyanuric acid
  • Very beneficial as a backup to chlorine generator systems
  • Ideal for use with secondary sanitizing systems like ozone, UV and AOP

In the swimming pool industry, one of the most popularly chosen forms for sanitizing water is liquid sodium hypochlorite. Commonly known as liquid chlorine, it is widely used in swimming pools both commercial and residential. It is a bit of a misnomer to use the term liquid chlorine; technically speaking it can better be described as a chlorinating compound. It would more correctly be referred to as liquid bleach.

Sodium hypochlorite is made up of a blend of liquid chlorine, water and sodium hydroxide. The popularity of liquid sodium hypochlorite is primarily due to its availability, cost and efficacy. Also, sodium hypochlorite will not contribute to an increase in calcium or cyanuric acid. It remains one of the purest and simplest formulas for the disinfection of bacteria and the prevention and removal of algae in swimming pools.


The discovery of chlorine as an agent for bleaching goes as far back as 1787 when a French chemist named Claude Berthollet developed a weak sodium hypochlorite solution by passing chlorine gas through sodium carbonate (soda ash). The product he created was known as Eau De Javelle or Javel Water. The name came from the region near Paris where it was manufactured. In 1820 Antoine Labarraque discovered that using sodium hydroxide with chlorine gas sodium hypochlorite could be produced much cheaper than with soda ash. Labarraque discovered many of the disinfectant properties of liquid sodium hypochlorite and is largely responsible for promoting its use worldwide.

RELATED: Key Misunderstandings and Problems in Pool Water Chemistry (Part I)

Early uses of sodium hypochlorite liquid bleach included the disinfection of medical facilities and hospitals, preventing the spread of disease from corpses; slaughterhouses; and food production. Much later in the early 1900s, liquid sodium hypochlorite would be incorporated into drinking water treatment. The use of chlorine disinfection at water treatment plants was very effective in exponentially reducing fatalities from waterborne diseases such as cholera and typhoid fever.


Since the “hypochlorites” had proven effective in the disinfection of waterborne diseases in drinking water, it was only natural that chlorination would make its way to swimming pools.

Both calcium hypochlorite and liquid sodium hypochlorite had been the two main treatments available other than chlorine gas. In particular, liquid sodium hypochlorite was preferred at public pools because it was safer to store than either chlorine gas or calcium hypochlorite. As pools began to grow in popularity beginning in the 1920s, the use of sodium hypochlorite found a new and growing industry. In the mid 1940s, the polio panic had people afraid to go to public pools. Liquid sodium hypochlorite became very popular at this time as a disinfectant because of chlorine’s proven ability to kill polio, and public pools began using it to help win back a fearful public.

As backyard residential pools began to gain popularity in the 1950s and ’60s, it was liquid sodium hypochlorite that played the key role in disinfecting and keeping the pools clear. Before chlorine tablets, feeders and floaters began to make their way into backyard pools there was basically a simple routine. Sanitize with liquid sodium hypochlorite pH with muriatic acid. Moving into the 1970’s, a newer chlorine compound was beginning to show up in backyard swimming pools. The new kid in town was the stabilized chlorine tablet known as trichlor.


While calcium hypochlorite and liquid sodium hypochlorite were enjoying mainstream popularity in backyard pools, there was a problem: Both forms of hypochlorite were unstabilized. In the summer sun, the free chlorine created by these unstabilized compounds didn’t last very long. In fact, nearly all the chlorine from liquid sodium hypochlorite could be destroyed in about four hours by direct summer sunlight. As this problem became more widely recognized, pool owners and operators began adding extra amounts of liquid sodium hypochlorite in the late afternoon or evening to replace losses due to sunlight earlier in the day.

RELATED: Key Misunderstandings and Problems in Pool Water Chemistry (Part II)

In 1956 Monsanto began to produce and distribute cyanuric acid. At levels of 30 ppm, CYA helped free chlorine residuals last up to eight times longer than without the additive. There was an incredible benefit from adding CYA to the pool water. In the late 1960’s and early 1970’s solid tablets of stabilized chlorine known as isocyanurates began to make their way into residential swimming pools.

The selling point was convenience for the owner. The tablets could be added to an inline feeder or a floating container known as a floater. The advantage was that the stabilizer to make chlorine last was built right into the tablet. Voilà: instant convenience for the modern pool owner. As this system of treating backyard pools grew in popularity liquid sodium hypochlorite was pushed out as the main source of pool sanitization. Liquid was now mainly used as a backup or shock to trichlor tablets in pools.


While the two-in-one convenience of trichlor tabs continued to grow in popularity (an 8-oz. trichlor tablet contains over 54% CYA by weight, with the other 46% being chlorine) there was something going on beneath the surface that pool pros were beginning to observe. Early on there was a perception that higher levels of CYA could lock up the effectiveness of the chlorine. This was known as “chlorine lock,” and whether it was mythical or factual it was the source of many a bar room brawl. From the halls of academia to the discussions of field techs, the debate over CYA has raged for over 30 years.

One thing that began to become certain on an anecdotal basis was that pools that used trichlor tabs with increasing amounts of CYA were becoming a struggle to maintain. It seemed more of a challenge to keep free available chlorine levels and many pools had algae problems toward the end of the season. Trichlor tab users began to become aware of some side effects from use that led to complications in water treatment. The CYA being released from the continual dependence on trichlor tablets was building up and impeding the effect of the chlorine.

Despite these issues, there remained a large contingent of folks under the impression that more is better no matter what, while others were beginning to see the need to drain and dilute to keep levels down. Regardless of the stance on the CYA debate one point was clear: Trichlor tabs made the maintenance of pool water more complicated. Some other ancillary considerations included the fact that trichlor tabs were very acidic, which leads to using more soda ash to balance pool water. Then came the discovery of how CYA interferes with total alkalinity readings if the levels are higher than 60 ppm. Another adjustment to be made. Next came emerging research from the Centers for Disease Control pointing out that in the cases of crypto remediation, it takes a lot more chlorine and contact time to inactivate the parasite if CYA levels are present even as low as 15 ppm.

RELATED: Water Chemistry: Lessons Learned

Over time, reports emerged showing there must be a proper ratio between the ppm of free chlorine and CYA present in the pool. Some suggest using a ratio of 7.5% ppm of free chlorine times the CYA ppm level. For example, if CYA level is 60 ppm it would take 4.5 ppm of residual chlorine to kill bacteria and prevent algae growth. 60 x 7.5%=4.5 ppm. The latest report from the CDC and Model Aquatic Health Code ad hoc committee on CYA suggest a CYA to chlorine ratio of 20:1 in order to effectively inactivate bacteria. That means at 20 ppm CYA there must be 1 ppm of FC in order to effectively inactivate bacteria such as E. coli or giardia. While the debate on all of this continues, it is clear to see that the perceived convenience of trichlor tablets has led to a lot of complications for both service pros and pool owners.


Moving into the ’90s, sanitizing devices such as salt chlorine generators began to become more popular. These were sold to many new pool owners looking for a “non-chemical” pool that relied only on salt to purify the water. Many of those new pool owners were not aware that the salt unit installed on their new pool was basically a small liquid sodium hypochlorite factory.

Liquid sodium hypochlorite is made at chemical plants by first using a process of electrolysis to split the sodium chloride molecule. Simple salt is divided into chlorine and sodium hydroxide then this is blended with good old H2O to form liquid sodium hypochlorite. The salt water generators on swimming pools do the same thing on a smaller scale. So, they really are more correctly referred to as chlorine generators.

Like any type of chlorine sanitized swimming pool, a chlorine generator pool will still need to have a level of CYA to prevent rapid burn out of chlorine from the UV rays of the sun. CYA levels in a salt pool are good at a level of 30 - 50 ppm. Using the 7.5% ratio, with a CYA of 30 ppm, the ppm of FC needed from the chlorine generator would be 2.25 ppm. At times a chlorine generator may need a backup of manually added chlorine to keep the water quality good. This could be a result of heavy swimmer load, equipment failure or power outage. Liquid sodium hypochlorite is one of the best backup sanitizers for chlorine generator systems. The main reason for this is due to the byproduct that comes from the use of sodium hypochlorite:

Different types of chlorine and their byproducts:

  • Calcium hypochlorite: byproduct left in water is calcium
  • Trichlor: byproduct left in water is CYA
  • Liquid Sodium Hypochlorite: byproduct left in water is sodium chloride (salt)

Out of all the types of chlorine, liquid sodium hypochlorite gives a chlorine generator pool what it needs to function properly — more salt. Liquid sodium hypochlorite is a great additive to a salt chlorine generator system because it can help to replace the salt that can be lost from backwashing or splash out of water.


There are secondary sanitizer devices which have gained in popularity recently: ozone, UV and advanced oxidation process. All these systems are not approved by the EPA as primary sanitizers. Even though they oxidize and inactivate many pool water contaminants, they are unable to leave a measurable residual in the water. That means that chlorine still needs to be the primary sanitizer in these pools with a residual between 1-4 ppm.

RELATED: New Thinking: Chlorine/Cyanuric Acid In Balance

CYA levels need to be controlled in these types of systems in order to get the desired residuals to ensure protection from bacteria in the pool. 30-50 ppm of CYA is the recommended level in these systems. At a CYA level of 60 it would take 4.5 ppm of chlorine to inactivate bacteria. This is outside the required EPA rule. Since the main purpose of secondary devices is to allow for complete disinfection with lower amounts of chlorine, the use of trichlor in these types of systems would not be a good fit. Again, liquid sodium hypochlorite is a preferred choice for these systems because it provides manageable levels of free chlorine without byproducts that will reduce the system effectiveness of UV, ozone or AOP.


With all the chatter, research and articles of late regarding CYA also comes loads of confusion and questions for service techs and pool owners. Ongoing research will no doubt uncover further benefits and detriments of using CYA and trichlor in swimming pools. Amidst all this, there is still a reliable, safe and simple way to get chlorine sanitizer with a measurable residual into swimming pools. Liquid sodium hypochlorite has been proven throughout the history of pool chlorination to be one of the most cost effective, easiest and safest ways to disinfect pool water. Perhaps this is a good time to return to the simple.

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